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Systematic Mapping of Bedrock and Habitats along the Florida Reef Tract—Central Key Largo to Halfmoon Shoal (Gulf of Mexico)

USGS Professional Paper 1751

by Barbara H. Lidz, Christopher D. Reich, and Eugene A. Shinn

Introduction: Table of Contents Project Overview Project Objective Geologic Setting Primary Datasets Primary Products - Overview Maps & Evolution Overview: Introduction Depth to Pleistocene Bedrock Surface Reef & Sediment Thickness Benthic Ecosystems & Environments Sedimentary Grains in 1989 Summary Illustration Index Map Evolution Overview Tile-by-Tile Analysis Organization of Report Tiles: 1, 2, 3, 4, 5, 6, 7/8, 9/10, 11 Summary Acknowledg- ments References Disclaimer Related Publications

Summary Summary Modern Reefs and the Rock Record Limestone Compaction Ancient Reefs Fate of the Present Coral Reef Ecosystem Limestone Compaction: Carbonate buildups have long been a focus of intense geologic study but initially for reasons other than the ecosystem decline and environmental concerns that are paramount today. Geologists originally studied modern reefs mainly as an aid to understanding the distribution and life requirements of ancient reefs. An underlying reason is that many ancient reefs are significant hydrocarbon (oil and gas) reservoirs. Various types of reefs, including those made of coral, have existed for hundreds of millions of years, and the majority lies buried in the subsurface. Globally, ancient subsurface coral reefs and other types of organic-carbonate buildups that have undergone sufficient overburden (burial) and thermal maturation (pressure) through time (tens of millions of years) have become reservoirs of the hydrocarbons we use today. Most hydrocarbons are trapped in porous and permeable limestone that has a three-dimensional structure and that escaped complete marine and later groundwater or freshwater cementation. Structural hydrocarbon traps include impermeable overlying strata, buried salt domes, and fault blocks, to name a few. Oil and gas have long been pumped from such structures along a 1,000-mile trend of Cretaceous reefs (~144 to 66 Ma, Fig. 7B) along the Gulf of Mexico coast. This trend includes the producing Sunniland Field (Fig. 116) as well as 13 other producing wells in Florida. Thus, early incentives to study modern reefs, along with the few ancient reefs exposed by mountain-building processes and erosion, were driven mainly by economic concerns. Of particular interest was the question of how the high porosity (up to 50% of any living coral reef is void space) can be preserved after burial. USGS studies of modern coral reefs using seismic instruments and portable underwater coring devices were initially aimed at unveiling the factors that control reef distribution, thickness, and porosity. Porosity was also examined in laboratory experiments. Hydraulic presses were used to squeeze or compact lime muds from Biscayne Bay and skeletal coral sands from Rodriguez Key to simulate burial and determine the amount of porosity that might survive and become a reservoir for fluid (Figs. 137, 138A, 138B; Shinn et al., 1977b, 1978; Chanda, 1978; Shinn and Robbin, 1983). The compaction studies were also aimed at discovering clues derived from the internal structure of modern muds and reefs to recognize how changes in sediments and fossils occur over time. Figure 137. An "H-frame" hydraulic press with small compression chamber (wrapped in a black electric heat jacket) in place beneath a ram (hydraulic piston) was one of the types of presses used in compaction experiments (from Shinn and Robbin, 1983). With a small chamber, pressures as high as 3,060 kg/cm2 (43,476 psi) were easily obtainable (43,476 psi is approximately equal to 15,000 m, or 43,476 ft of overburden). A string attached a clock-driven drum with ink stylus mounted on top of the frame (out of view) to the ram to make a continuous compaction record. A centimeter ruler was also attached vertically at the right of compaction chamber and was read by a stylus attached to the ram. psi = pounds per square inch. [larger version] Figure 138. (A) Uncompacted core from Rodriguez Key bank showing abundant sea grasses, an open burrow, and a mollusc shell near the bottom (from Shinn and Robbin, 1983). Circular grains are cross sections of the coralline alga Goniolithon. Flattened grains are Halimeda plates. The core was impregnated with resin before slicing. (B) Compacted core from Rodriguez Key bank; sediment was compressed under an extreme load (952 kg/cm2, or 13,568 psi) for 34 days (from Shinn and Robbin, 1983). Note mashed wavy organic matter from sea grasses, obliteration of open burrows, complete absence of recognizable sea grasses, and closer packing of skeletal grains. Chemical compaction (organics dissolved under pressure) occurred in this core. psi = pounds per square inch. [larger version] The compaction studies replicated sedimentary features commonly found in ancient fine-grained limestone. Included (Fig. 138) were (a) flattened burrow fillings; (b) mashed or obliterated fecal pellets; (c) obliterated small voids and identifiable sea grasses; (d) reorientation of fossils toward the horizontal; (e) minor breakage of fossils; and (f) creation of wispy organic layers that often drape over fossils or other less compactable inclusions. Shinn and Robbin (1983) showed that lime sediments with marine pore waters can readily compress to at least half their depositional thickness and approximately half their original porosities under as little as 100 m (328 ft) of simulated overburden (burial). Significant compaction resulted from pressures simulating less than 305 m (1,000 ft) of burial. Minor chemical compaction (pressure dissolution, evidenced by thin sections showing quartz grains piercing mollusc shells without causing fractures) was observed in cores compacted at room temperature to a simulated overburden of 4,136 m (13,568 ft) for as little as 4 days. Given geologic time, significant chemical compaction probably occurs at depths much shallower than 4,100 m. continue to: Ancient Reefs

Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751